Certain polymers can be excited by electric, chemical, pneumatic, optical, or magnetic field to change their shape or size. For convenience and practical actuation, using electrical excitation is the most attractive stimulation method and the related materials are known as electroactive polymers (EAP) and artificial muscles. One of the attractive applications that are considered for EAP materials is biologically inspired capabilities, i.e., biomimetics, and successes have been reported that previously were considered science fiction concepts. Today, there are many known EAP materials. Some of the EAP materials also exhibit the reverse effect of converting mechanical strain to electrical signal allowing using them as sensors and energy harvesters. Efforts are made worldwide to turn EAP materials to actuators-of-choice and they involve developing their scientific and engineering foundations including the understanding of their operation principles. These are also involve developing effective computational chemistry models, comprehensive material science, and electro-mechanics analytical tools. These efforts have been leading to better understanding the parameters that control their capability and durability. Moreover, effective processing techniques are developed for their fabrication, shaping, electroding, and characterization. While progress have been reported in the research and development of all the types of EAP materials, the trend in recent years has been growing towards significant development in using dielectric elastomers.
Manipulation, mobility, and activation of engineered mechanisms and systems are done by actuators, typically consisting of motors, gearboxes, and associated mechanisms. These are hard, heavy, and noisy in contrast to muscles of biological systems, which are soft, light weight, and quiet. However, living muscles are driven by a complex microscopic ionic molecular linear motor mechanism that is very difficult to mimic and currently impossible to manufacture. We can emulate muscle action in a soft mechanism using certain polymers that can be stimulated by electric, chemical, pneumatic, optical, or magnetic field, to cause shape or size change. Polymer actuation using electricity is convenient and practical. Such electroactive polymers (EAP) are among the closest to emulate biological muscles . As polymers, they have many advantages including mechanical flexibility, low density, as well as being easy to process, and mass produce. In addition to their mechanical response to electricity, some of the EAP materials also exhibit the ability to sense mechanical strain and to harvest electrical energy from it (e.g., [3, 27, 28, 75, 118, 119, 121]).
These characteristics are making them highly attractive for use in muscle-like actuators. Some are biologically inspired (i.e., biomimetic applications) [8, 9, 11], and all can function without hard metallic gears and mechanisms. Examples of the applications of EAP actuators include a polypyrrole fish , ionic polymer–metal composite robot fish , miniature dielectric elastomer grippers for satellites , ionic conductor loudspeakers , an electrostrictive polymer catheter , a haptic interface , active braille displays , a fish-like blimp , static electric rotary motors , worm-like robots , a crawling robot with no hard electronics , facial animatronic devices , optical devices , biomedical devices, microfluidic devices, and even a wearable device to assist eyelid blinking . The impressive improvements in the field [52, 68] are increasingly attracting the interest of engineers and scientists from many different disciplines.
Many EAP actuators are still emerging and need further advancement in order for them to form part of mass-produced products. This requires the use of computational chemistry models, comprehensive material science, electro-mechanic analytical tools, and material processing research. To maximize their actuation capability and durability, effective fabrication, shaping, and electroding techniques are being developed. In addition, techniques of characterizing their response as well as documenting them in databases and related standards  are being established. Engineers are continually seeking to find niche critical applications for these materials to enable them to see daily use as part of mass-produced products.
The development of EAP actuator mechanisms—–historical overview
The first to conduct a documented experiment with EAP materials was , who demonstrated electroactive strains in a rubber sheet by spraying electric charge onto it. The spraying of electric charge onto a dielectric was more recently repeated by , who demonstrated how this electrode-free method could be used for eliciting large strains in a simple actuator without suffering dielectric breakdown.
 is credited as the first to formulate the effect of the strain response to electric field activation in polymers.  discovery of the piezoelectric polymer called electret is the next important milestone in the field of EAP. He produced the material using rosin (carnauba wax) and beeswax that was solidified by cooling while being subjected to a DC bias field. Electrets are electroactive where they deform under electric field and produce electric field when deformed. Since the electric field generates quite small strain in electrets, their application has been limited to sensors.
The responsive gels were pioneered with the development by Katchalsky and his co-investigators in Israel . They reported chemo-mechanical activation in gel polymers causing shrinkage or swelling in the presence of acid or alkaline, respectively. Important milestone studies of responsive gels and their electro-chemical activation have taken place at the Hokkaido University, Japan . The developed gel polymers were demonstrated to create large strain under relatively low activation voltage .
The discovery of electrets was followed with Fukada’s work on piezoelectric biopolymers  and Kawai’s discovery of significant piezoelectric activity in polyvinylidene fluoride (PVDF) . The investigations of PVDF and its copolymers have shown strong electromechanical activity in certain noncrystalline polymers with very large dielectric relaxations resulting from orientation of their molecular dipoles (e.g., [49, 120]). The development of PVDF as an electroactive polymer was followed with extensive search for other polymer systems that exhibit significant response. Mostly during the 1970s and 1980s, extensive investigations related to PVDF have taken place in efforts to improve the performance and seeking applications . The limited strain that PVDF produces led to its use mostly as sensors and ultrasonic wave transducers .
Successes in developing effective new EAP materials were reported mostly in the 1990s and examples include [14, 16, 65, 76, 78, 128]. The use of stretchable electrodes to place electrical charge on the surface of a rubbery dielectric, an invention by workers at SRI, led to the creation of the dielectric elastomer (DE) actuator. These workers demonstrated strains that exceeded 100% with a relatively fast response speed (< 0.1 s) .
In 1995, the lead author started his research in the field of EAP and soon he determined that in order to accelerate development of EAP materials and lead to effective actuators it is critical to form worldwide cooperation. Therefore, he initiated various forums for exchanging information including the SPIE annual EAP Actuators and Devices (EAPAD) Conference that started in March 1999 as part of the SPIE Smart Structures and Materials Symposium . In 1999, the lead author posed an arm wrestling challenge (http://ndeaa.jpl.nasa.gov/nasa-nde/lommas/eap/EAP-armwrestling.htm) in an effort to promote worldwide development towards realizing the potential of EAP materials. The challenge consists of having an EAP activated robotic arm win against human in a wrestling match. An example of one of the arm wrestling robots is provided in Fig. 1.
Choosing to focus on arm wrestling with a human was done in order to emphasize that human muscle is a baseline for performance comparison. Success in emulating human muscle will allow the use of EAP materials to improve many aspects of our life including the development of effective implants and prosthetics, active clothing, realistic biologically inspired robots as well as fabricating products with unmatched capabilities and dexterity.
Today, many workshops, meetings, and conferences are taking place that are covering the subject of EAP. As information archive, the lead author created the WorldWide EAP (WW-EAP) Webhub  linking the growing number of related websites. In addition, in June 1999, the lead author started publishing the web-based semi-annual WW-EAP Newsletter that provides snapshots of the advances in the field.
Mostly, EAP materials are still custom-made by researchers but there is a growing effort of various manufacturers to establish mass production methods and commercial products. In order to help making these materials widely available, the author established a website that provides fabrication procedures for the leading types of EAP materials and a website about sources for obtaining these materials and samples .
The two EAP actuator groups
Based on the mechanism that activates EAP actuators, they can be divided into two major groups including ionic and field-activated EAP. The actuation process of ionic EAP materials is involved with diffusion of ions [7, 86]. In addition, the material is in film form containing electrolyte and is covered by two electrodes. Examples of these materials include conductive polymers, ionic polymer gels, polymer-metal composites, and carbon nanotubes. Their advantages include operation at low activation voltage (1–2 V) and generating large bending displacements. Disadvantages include the need to maintain electrolyte and difficulty to sustain constant displacement under activation of a DC voltage (except for conductive polymers). In contrast to the ionic EAP, the field-activated (electronic) EAP materials are driven by Coulomb forces and this can require high voltages (> 10-V/μm) [7, 24]. This EAP group is stimulated by the electric field between the electrodes that are applied onto the polymer in the form of a film. The field-activated type of EAP materials holds the activated displacement when operated by a DC voltage, which is a great benefit to many applications including robotics. In addition, they have higher mechanical energy density and they can be activated in air with no constraints. However, they require high activation field that may be close to the level of dielectric breakdown level. A summary of the advantages and disadvantages of the two EAP material groups is given in Table 1.
Field-activated EAP actuators
Two of the principal causes for electrostriction in an electric field polymer actuator are associated with (1) intrinsic field-induced molecular conformational changes to the elastomer (ferroelectric polymers) and (2) extrinsic electronic charge attraction-repulsion at the electrodes on the surfaces of the actuator. While both phenomena are present to a greater or lesser extent in all actuators of this kind, the dominant mode of actuation will be one or the other. Thus, they fall into two main types: ferroelectric polymer actuators and dielectric elastomer actuators.
Ferroelectric polymer actuators
Ferroelectric polymers are characterized by an electrical polarization that can be changed in an applied electric field . The external field will apply moments to polarized groups within the polymer. The most widely known ferroelectric polymer is the Poly(vinylidene fluoride), also known as PVDF or PVF2 [12, 34, 79, 104], and its copolymers .
These polymers have partial crystallinity with an inactive amorphous phase. In 1998, Zhang and his coinvestigators used electron radiation to introduce defects into the crystalline structure of the copolymer P(VDF-TrFE) and observed increase in the dielectric constant. The resulting material generates strains as large as 5% and levels of pressure of about 45 MPa under voltages of about 150 V/μm. The drawback to the irradiation is the introduction of many undesirable defects and formation of cross-linking and chain scission . By producing terpolymers via molecular design, the issue was addressed and the degree of conformational changes at the molecular level was enhanced. The advantage of terpolymers is that they generate higher electro-mechanical response than the high energy electron irradiated copolymer [129, 130]. To increase this constant, a composite material was proposed, using filler that is made of a high dielectric constant (the 2001 edition of ). This approach was successfully implemented by Zhang and his coinvestigators  who used an all-organic composite that consists of particulates having high dielectric constant (K > 10,000). Photographs of such a composite ferroelectric EAP in passive and activated states are shown in Fig. 2.
Electrostrictive graft elastomers, another ferroelectric type, consist of a chain structure (i.e., backbone) with molecular pendant group that can alter alignment due to polarization from the external electric field. These polymers may consist of two components, a flexible backbone macromolecule and a grafted polymer made of a polarizable molecular or nanocrystalline structure. Subjecting such elastomers to large electric field was reported to produce about 4% strain and about 24 MPa stress [112, 113, 129]. A combination of the electrostrictive-grafted elastomer with a piezoelectric poly(vinylidene fluoride-trifluoroethylene) copolymer allows producing various compositions of ferroelectric-electrostrictive molecular composite systems. Such combinations can be operated as piezoelectric sensor as well as electrostrictive actuator. The photographs in Fig. 3 show on the right an activated grafted elastomer-based bimorph actuator while on the left is the EAP at the rest state.
Recent developments in ferroelectric type actuators include a P(VDF-TrFE) for micropump applications .
Dielectric elastomer EAP actuators
Dielectric elastomers actuators are currently the subject of wide research interest. Their mechanism of actuation involves the placing of electric charge on the surfaces a thin rubbery dielectric (Fig. 4). Earlier, we described the experiment of  who sprayed electric charge unto a rubber membrane and this resulted in a shape change to the membrane and how the charge spraying method of Roentgen was repeated more recently by , demonstrating how large electroactive strains could be produced using this approach. The practical application to actuation became possible in the 1990’s when workers at SRI (Menlo Park Cal.) applied the surface charge using a flexible electrode material that coated the surface of thin dielectric materials . These materials included a stretchable acrylic and silicone rubber. Electroactive strains of greater than 100% were achieved . The mechanism for actuation was surface electrostatics mathematically described as the Maxwell Pressure (Eq. 1) that has been experimentally validated by several researchers including ([56, 88]; and ):
where the Maxwell pressure σMaxwell is calculated from the absolute and relative permittivities, the voltage V and the membrane thickness, d. This simple equation that relates Maxwell stress to field strength is at the heart of the analysis of strain. However, given that the strains are large, the question of how much deformation strain will occur under a particular field strength will be governed by non-linear material properties of elastomers. This complex issue is discussed in the analysis by . Other workers have shown how extremely large actuation strains can be achieved through elimination of electromechanical instability (this instability is manifested through formation of wrinkles and dielectric breakdown) by lateral pre-stretch [57, 58].
Commercial uptake of DE is being preceded by a search for better materials for dielectric and electrode, ways of improving performance, and means of manufacturing . For the past two decades, the common dielectric material for prototyping and testing new DE concept actuators has been the 0.5 mm and 1-mm thick very high bond (VHB) acrylic tapes from 3 M: VHB 4905 and 4910 respectively. The use of VHB in laboratories for making DE actuator demonstrators is gradually being replaced by silicone-based elastomers (polysiloxanes) . The dielectric constants for off-the-shelf silicones are relatively low (2–3), roughly less than half some of the values measured for VHB . However, these elastomers do not suffer the high viscoelasticity of VHB and, therefore, can demonstrate fast response with excellent reliability for millions of cycles. Actuators built from silicone elastomers are also less fragile than those built from VHB and can demonstrate vastly better reliability over time and millions of cycles. Research is currently focused on producing silicone elastomers with high dielectric constant for high field while maintaining low modulus for good actuation and high dielectric strength. Two methods for doing this include blending in polymers with high dielectric permittivity or using chemically modified silicones. The latter is achieved through grafting organic dipoles to the silicone elastomer backbone. For reviews of how polysiloxanes have been modified see . In another more recent review by , the opportunities from chemical modification to polysiloxanes as well as other polar elastomers are reviewed and clear directions have been put forward for workers in this area with the end goal of producing actuators that are operational at very low voltages (~ 24 V).
For electrode materials, laboratory workers have used several forms of carbon particles suspended in silicone grease. This has been relatively easy to apply and effective for fast prototyping of actuators but suffers the drawback of being messy and difficult to protect from damage. The review of DE electrodes by  describes many advances in this area that now include loose carbon powder stamped onto the surface, surface printing a carbon in a fluid, formation of carbon/silicone composite electrodes, and deposition of compliant thin-film metal electrodes. The latter can be made compliant by the formation of micro-wrinkles due to buckling induced by DE compression or through metal deposition on a micro-corrugated silicone surface.
A number of DE actuator mechanisms have been demonstrated over the past 20 years . They involve either in-plane expansion of a stretched membrane (Fig. 5) with in-plane electrode expansion that can also lead to out-of-plane displacement with some biasing mechanism (air pressure, spring, etc.). On the other hand, out-of-plane contraction usually requires the stacking of many membranes in order to provide a large enough contraction (Fig. 6). Reviews of DE actuators can be found in (  [3, 99. 17]) and the book edited by . Some recent research interest includes the use of an encased ionic fluid and/or the fluid within which the DE robot swims [26, 63], non-linear springs coupled to DE for enhanced actuation [40, 41], lab-on-a-chip devices for peristaltic pumps , haptic feedback  and cell stretchers , optical applications (including tunable grating, lenses, laser speckle reducers, and tunable windows) [37, 93, 107], coupling actuators with piezoresistive switches , and hydraulically amplified self-healing electrostatic actuators (HASEL) . The latter use a contained fluid dielectric that is pumped out from between the electrode plates. Electrostatics can also be combined with DE actuation to enhance the ability to grip and handle objects [39, 106]. As an aside, the electrostatic mechanism has recently been combined with the principles of origami to produce a new type of high strength, high displacement ribbon actuator .
Sophisticated means for DE actuator fabrication are also under development that can be used for mass-production . These include stack actuator fabrication from pre-existing commercially available dielectric membrane material , spin coating , and screen printing of the electrode .
Ionomeric polymer-metal composites
The ionomeric polymer–metal composites (IPMC) have been widely studied as an ionic EAP material . In 1992, three different groups of researchers independently reported the development of IPMC as an EAP material including  in Japan, as well as [101, 105] in the USA. The attractive characteristic of IPMC is the significant bending in response to a relatively low electrical voltage (Fig. 7), where the base polymer provides mobility channels for positive ions to migrate through fixed network of negative ions on interconnected clusters [77, 86]. The response of IPMC is relatively slow (< 10 Hz) because of the need for ions to physically travel though the polymer.
In recent years, research related to IPMC has been focused on improvements in the modeling [2, 80] and fabrication methods [22, 52, 110] as well as the performance as actuation material . These are done as part of the efforts to understand how IPMC materials function and to enable broad range of IPMC based designs and applications. Using 3D manufacturing, researchers are seeking to create shapes that are not possible with commercially available ionomer options. The development of the related processes is involved with great challenges since the Nafion, which is the leading base material of IPMC, needs to be kept hot during the printing process in order to assure that each of the layers are kept adhering to each other. Researchers at the University of Nevada, Las Vegas, and their collaborators have made significant progress in addressing the related challenges .
Conducting polymer EAP materials offer mechanical energy densities of over 20 J/cm3 that is relatively high and have the potential of highly effective actuation materials . A sandwich of two conducting polymer electrodes (e.g., polypyrrole) with an electrolyte between them forms a bending EAP actuator. They typically function via reversible counter-ion insertion and expulsion that occurs during redox cycling [4, 84, 85, 87, 102, 108]. Voltage applied between the electrodes causes oxidation at the anode and reduction at the cathode and result in a volume change mainly due to exchange of ions with the electrolyte . The electric charge is balanced by migration of ions between the electrolyte and the electrodes. The added ions cause swelling of the polymer while their removal results in shrinkage and therefore bending of the sandwich. Conducting polymer actuators require voltages in the range of 1–5 V (Fig. 8) and the speed increases with the voltage. The efficiency of conducting polymers is relatively low (~ 1% if no electrical energy is recovered) .
Advances in conducting polymers (CP) over the last two decades have been made resulting from the efforts to apply novel modeling and fabrication techniques [44, 68]. Nanocomposites of polymers, carbon nanotubes, graphene, and inorganic compounds have been incorporated to obtain special structure and properties. In addition, progress has been made in applying conducting polymers as sensors and actuators with potential biomimetics applications. The capability of CP in the form of trilayers has been improved to the level that researchers are considering it as potential alternative to piezoelectric and electrostatic actuators . Efforts are also continuing towards making all solid-state CP actuators and recent progress has been reported by Ribeiro and his research collaborators . In addition, Tan and his collaborators used solid polymer electrolyte sandwiched between two poly(3,4-ethylenedioxythiophene) (PEDOT) electrodes and fabricated and characterized the produced actuator.
Carbon nanotubes (CNT) consist of nanometer-size tubes that are able to produce strains of about 1% . The use of carbon nanotubes as EAP was first reported in 1999 ([16, 94, 109]). Compared with other types of actuators, carbon nanotube (CNT) actuators have the potential to produce very high work/cycle and this possibility is the result of their very high Young modulus. However, this potential of high energy density and force has not been realized yet in devices at the macro-scale yet. Using CNT as actuator can be made to operate as a bending device using a layered structure or in a single sheet/fiber. As a bender, a carbon nanotube actuator can be constructed by laminating two narrow strips of carbon nanotube sheet with an electrically insulated intermediate adhesive layer. The resulting three-layer strip is then immersed in an electrolyte. Application of ~ 1 V is sufficient to cause bending, and the direction depends on the polarity of the field with a response that is approximately quadratic relationship between the strain and charge. The carbon–carbon bond in nanotubes (NT), which are suspended in the electrolyte, and the change in bond length are responsible for the actuation mechanism. A network of conjugated bonds connects all carbons and provides a path for the flow of electrons along the bonds. The electrolyte forms an electric double layer with the nanotubes and allows injection of large charges that affect the ionic charge balance between the NT and the electrolyte (Fig. 9). The more charges are injected into the bond the larger the dimension changes. Removal of electrons causes the nanotubes to carry a net positive charge, which is spread across all the carbon nuclei causing repulsion between adjacent carbon nuclei and increasing the C–C bond length.
The advantages of CNT materials are quite attractive for many applications. This is the result of the CNT potential for highly effective supercapacitors, actuators, and lightweight electromagnetic shields . Advances in the development of CNT materials and their applications have been reported in all its aspects including synthesis, purification, and chemical modification . The interest in the commercial use of carbon nanotubes (CNTs) has been steadily growing worldwide reaching levels and it is now at the level of several thousand tons per year. The advances have enabled the integration of CNT materials in thin-film electronics and large-area coatings. Such products as rechargeable batteries, automotive parts, and sporting goods are being produced using bulk CNT powders. Development in graphene synthesis and characterization is expected to lead to significant improvement to their fabrication and performance and to lead to potential related commercial products in the coming years [29, 43].
Ionic polymer gels
Ionic polymer gels are generally activated by a chemical reaction where changing from an acid to an alkaline environment causes the gel to shrink or swell, respectively. This chemo-mechanical behavior was first reported in 1955 by Katchalsky and his co-investigators [47, 62]. The mechanical change results either from the displacement of water out of the gel, or the redistribution of water within the gel. Since the 1980s, researchers at the Hokkaido University, Japan,  have extensively studied ionic polymer gels towards developing EAP actuators. These studies were followed by numerous other investigations worldwide (e.g., [18, 19]). Generally, ionic polymer gels generate very large strains but with relatively low actuation force .
Limited progress in advancing the capability of ionic polymer gels has been reported in recent years. These include the use of ionic liquids combined with macromolecules  to enable making actuator that is less sensitive to open air. The researchers focused on the use of ionic liquid-based polymer electrolytes that consist of block copolymers and polyimides. These combinations were demonstrated to enable ionic polymer actuators that are easier to produce and have higher performance and durability.
Other polymer-based actuation materials
Twisted and coiled polymers
Twisted and coiled polymer actuators via twisting nylon threads or fishing wires have been a significant development in polymer-based soft actuators in recent years [15, 74, 124]. The activation is done thermally leading to sizeable deformation and high power/mass ratios. The material can be activated electrically by causing Joule heating. To address the need to control the response using feedback from anti-windup compensator,  developed an algorithm that they validated through numerical simulations and experiments, where the maximum overshoot and the setting time decreased by the effect of the anti-windup compensator.
Since the early 1990s, new EAP materials have been developed that generate large strains making them highly attractive for use in actuators. Their operational similarity to biological muscles, including resilience, damage tolerance, and ability to induce large actuation strains makes them unique compared with other electroactive materials. However, the application of EAP materials as actuators still involves many challenges.
Addressing these challenges requires continuing technology development and the growth in multidisciplinary cooperation among experts from various fields including chemists, materials scientist, roboticists, computer, and electronic engineers. Researchers are increasingly making improvements in the various related areas including better understanding of the operation mechanism of the various EAP material types. The processes of synthesizing, fabricating, electroding, shaping, and handling are being refined to maximize the actuation capability and durability. These include the development of 3-D printing of EAP as well as magnetically activated materials . Methods of reliably characterizing the response of these materials are being developed and efforts are being made to establish process standards and databases with documented material properties to support engineers that are considering the use of these materials. The general trend in recent years has been towards the application of dielectric elastomers and significant advances have been reported.
Applying EAP materials as actuators of manipulation, mobility, and robotic devices involves multidisciplinary efforts for new materials, chemistry, electromechanics, computers, and electronics. Even though the actuation force of the existing materials requires further improvement, there have already been successes in the development of mechanisms that are driven by EAP actuators. However, seeing EAP replace existing actuators in commercial devices and engineering mechanisms would require identifying niche applications where EAP materials would not need to compete with existing technologies. It is quite encouraging to see the growing number of researchers and engineers who are pursuing career in EAP-related disciplines. Hopefully, the growth in the research and development activity will lead to making these materials becoming the actuators of choice.
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WW-EAP Webhub: http://eap.jpl.nasa.gov
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Information about the process of making the leading EAP materials http://ndeaa.jpl.nasa.gov/nasa-nde/lommas/eap/EAP-recipe.htm
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Some of the research reported in this manuscript was conducted at the Jet Propulsion Laboratory (JPL), California Institute of Technology, under a contract with National Aeronautics and Space Administration (NASA). The authors would like thank Samuel Rosset, The University of Auckland, New Zealand, for his comments and suggestions that helped improve the paper. Also, the authors would like thank the many individuals’ who contributed to the field of EAP and apologize to those whose publications have not been referenced.
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Bar-Cohen, Y., Anderson, I.A. Electroactive polymer (EAP) actuators—background review. Mech Soft Mater 1, 5 (2019). https://doi.org/10.1007/s42558-019-0005-1
- Electroactive polymers
- Activatable polymers
- Biologically inspired technologies